You could look at these:

https://github.com/rgrinberg/opium https://github.com/inhabitedtype/ocaml-webmachine https://github.com/inhabitedtype/ocaml-session https://github.com/arenadotio/pgx (there are a few other as well, not sure which one would support PostGIS) https://github.com/Leonidas-from-XIV/slacko

When in doubt head over to https://discuss.ocaml.org/ and show an example if you run into any trouble.